A liquid crystal display (LCD) is a very popular type of display that is used in many electronic devices. The LCD display works on a fairly low voltage DC, typically about 3 to 5 volts, and may be formed of individual pixels that may be used to generate any type of alphanumeric characters or graphics. LCDs are relatively low priced and may be incorporated in any number of devices, such as cell phones, computers, calculators, watches, gas pumps, instruments, etc.
FIG. 1 shows a typical prior art LCD formed of a pair of substrates (glass plates) with a liquid crystal fluid therein and polarizers arranged on both sides. The glass or substrate is coated with a transparent metal coating which forms the electrodes of the display. The transparent metal coating may be any type of thin conducting material layer, such as gold, silver or tin. A popular choice is indium tin oxide (ITO). The inner surface of each plate is coated with a thin polymeric alignment layer, which is striated to give the liquid crystal molecules a preferred direction of orientation. Because the liquid crystal molecules are long-chain molecules, the striations cause the liquid crystal molecules to be aligned in a predetermined direction substantially aligned with the striations in the substrate. The LCD is formed with two substrates having striations running in different directions, such as 90 degrees, for example. The helical structure is therefore a rotation or twist of the liquid crystal molecules from one substrate to the other. In a nematic LCD, the twist is 90 degrees. In a supertwisted nematic LCD, the twist may be greater than 90 degrees.
As the polarized light from a first polarizer passes through an un-energized (off) display, it will be rotated 90 degrees by the liquid crystal and may be either absorbed or passed by the second polarizer. When the LCD is energized (on), the liquid crystal molecules will rotate in the direction of the electric field and no longer rotate the polarized beam of light. Again, depending on the direction of the second polarizer, the light will be either absorbed or passed. In the case of a positive image or reflective display, this would produce a dark character on a light background. The electric field therefore may be used to selectively block or transmit light. A display may be formed of a plurality of LCD pixels or alphanumeric symbols, with each pixel or symbol being an independently controllable LCD unit.
A drawback of LCDs is that, since they do not generate light, they suffer from a relatively poor contrast and are not highly visible. Therefore, LCDs require backlighting in order to provide high contrast and viewability. In a backlit LCD, a light source is positioned behind the LCD with the light passing through a diffuser, through the first polarizer, through the LCD itself, and then through the second polarizer. A prior art LCD must include a polarizer on both sides in order to function correctly. The diffuser is used to diffuse the light from the backlighting light source into the display itself. The diffuser is employed because prior art backlit LCDs typically use a non-uniform light source for backlighting. Without a diffuser, a prior art backlit LCD would therefore exhibit area luminance non-uniformity.
Popular reasons that LCDs enjoy increasing use include small size, low power consumption and low cost. LCDs are thin and light, relative to CRT type of displays. Therefore, laptop computer use is an area in which LCDs are widely used. Also, they require less power and are generally less expensive than other types of displays, such as CRTs, for example.
FIG. 2 shows a prior art backlight arrangement using a fluorescent lamp light source. Because the fluorescent lamp is a line light source, a diffuser is used to provide uniformity of light prior to input into the LC display. The fluorescent lamp is generally spaced apart from the diffuser in order to increase the diffusion of light without excessive losses. In addition, the spacing may reduce heating of the diffuser, and may prevent contact and damage due to shock or vibration of the display. This spacing contributes to the physical size of the display.
In another drawback of the prior art, the first polarizer is interposed between the diffuser and the LCD. The first polarizer is needed because the fluorescent bulb produces unpolarized light. The first polarizer polarizes the light entering the LCD.
FIG. 3 shows another prior art backlit LCD embodiment wherein, in order to reduce the dependence on the diffuser, the light source is a fluorescent bulb of a serpentine configuration. Alternatively circular or multiple line fluorescent sources have been used in order to attempt a more uniform light source. The diffuser and polarizers are still required.
FIG. 4 shows an edge lit LCD backlight configuration where the fluorescent light source is positioned along an edge of the display. A light pipe, such as an internally reflective wedge of acrylic, is used to diffuse the light and to conduct the light from the fluorescent bulb into the LCD display. The diffuser and polarizers are still required.
The prior art suffers from many drawbacks. Light loss is the largest drawback. First, the fluorescent lamp based system is inherently non-uniform. Therefore, a diffuser section is required in the prior art. However, the diffuser does not capture and redirect all light from the light source and therefore not all of the light generated by the light source actually enters the diffuser. In addition, a diffuser of the prior art, such as for avionics displays, passes only about 50 percent of the entering light. The diffuser section adds weight and thickness to the LCD display. Second, the prior art light source is unpolarized (the emitted light contains waves of all possible orientations) and a large amount of light is lost in the first polarizer. A typical polarizer, such as a film type used in LCDs, passes only about 38% to 40% of the entering light. A twisted nematic LCD requires the second polarizer to ensure that only the light of a desired orientation passes out of the LCD.
Therefore, in a prior art backlit LCD display, only about 4% of the light produced by the light source actually is transmitted completely through the LCD. The prior art backlit LCD is highly inefficient and most of the light produced for the backlighting is simply wasted. This results in a waste of energy, generation of unnecessary heat, etc.
Another prior art drawback is the size typically required for a fluorescent backlighting arrangement. Prior art avionics devices may add up to 4 inches in depth by including a fluorescent light source and a diffuser or light pipe.
Another drawback of the prior art comes from the nature of the fluorescent light source. Fluorescent light sources require high voltage, alternating current, and produce significant electromagnetic interference (EMI), and must produce a high light output in order to make the backlit LCD function properly and have adequate contrast and visibility.
What is needed, therefore, are improvements to light sources.